On Convex, there are 8 vector registers of 128 elements which are 64 bits
in precision. Obviously, the question is not how many are useful but what
is the optimal number. With 8 vector registers, we still do experience
vector register spills although that is dependent upon the code. Needless
to say, a vector register spill is much worse than a scalar register
spill.

No, I do not know of any published studies that state the optimal number
of vector registers. However, there are costs as more vector operators
are added. These costs are expansion of instruction format, more complex
micro code within the vector unit, more complex chaining analysis in both
the compilers and vector unit, etc... Ultimately, this cost is paid for by
the custumer.